A typical cellular radio system comprises a fixed base station network in which a subscriber terminal communicates with one or more base stations. The base station forwards traffic originating from the subscriber terminal and traffic to the subscriber terminal. In addition, the cellular radio system comprises e.g. a base station controller which controls the base stations, and a mobile services switching center. Typically, the cellular radio system also comprises a vocoder encoding a signal.
The vocoder may be placed in e.g. a TRAU unit (TRAU=Transcoder/Rate Adaptor Unit). The TRAU unit is placed in e.g. the base station controller or the mobile services switching center. The TRAU unit source decodes the signal and adapts the signal transfer rate to be suitable for the transfer network such as a PSTN network (PSTN=Public Switched Telephone Network). The TRAU unit generates TRAU frames which it transmits to the base station. The vocoder encodes e.g. speech. Encoding reduces the data rate of the signal in e.g. the transmission line to the base station. The vocoder and the base station transmit and receive data packets from one another. The data packets form TRAU frames that are transmitted at intervals of e.g. 20 ms. The frames formed are for example 320 bits in length. The frames contain for example encoded speech, control information, signaling, or data.
The vocoder is used in adapting the signal transfer rate, which means that the vocoder encodes e.g. a 64 kbit/s signal from a PSTN network to a 16 kbit/s signal. The vocoder functions as either a variable rate or a fixed rate device. Adapting the transfer rate of the signal enables optimizing the traffic capacity on the transmission line. Optimizing the traffic capacity, in turn, enables capacity reduction particularly during a soft handoff in which a subscriber terminal in a cellular radio system has a simultaneous connection to more than one base stations. The TRAU unit operates as a source encoder reducing the bandwidth of the Air interface between the base station and a subscriber terminal.
A cellular radio system with e.g. a fixed rate vocoder utilizes discontinuous transmission to reduce interference and to lower the power consumption of the subscriber terminal. This method is used e.g. by the GSM system. In a discontinuous transmission mode, the speech encoder of the subscriber terminal detects silence in the speech, during which the subscriber terminal at times (in periods of 480 ms) only transmits a silence descriptor frame, i.e. a SID frame (SID=Silence Indication Detection). The SID frame is typically used for generating comfort noise at the receiving subscriber terminal. If noise at a suitable level were not generated, the receiving party would probably perceive the silence as unpleasant. In the worst case, the receiving party might think that the connection was lost. For example, in the GSM cellular radio system one 320-bit-long TRAU frame is encoded and transmitted over the air at 20 ms intervals, whereby the transmission rate will be 16 kbit/s. By contrast, the transmission rate during silence may be e.g. 667 bit/s.
During discontinuous transmission, Voice Activity Detection (VAD) is carried out in the cellular radio system. In practice, the detection is performed in the TRAU unit. In addition, the detection may be done in the subscriber terminal. During pauses in speech, the encoder shifts to DTX mode, whereby only SID frames are transmitted. The SID frames transmitted during pauses in speech contain updating information which is used for updating the aforementioned noise. The receiver uses the updating information in the generation of noise. Thus, the TRAU frames transmitted during pauses in speech typically only contain updating information which is transmitted at intervals of 480 ms. According to the prior art solution, however, the utilization factor of the capacity on e.g. the transmission path between a base station and a vocoder is not optimal, especially during pauses in speech.
A TRAU frame may vary in length considerably. Typical frame lengths include 268, 126, 56 and 22 bits. Also, typical frame lengths include 172, 80, 40 and 16 bits. The frames are furthermore transmitted at e.g. two different rates. A TRAU frame typically includes a 17-bit-long synchronization word. The synchronization word begins with 16 `0` bits and one `1` bit. According to the prior art solution, the end part of the frame in a DTX situation is filled with spare bits. As spare bits, e.g. `1` bits have been employed.
If the cellular radio system includes a variable rate vocoder, the end parts of the frames have been filled with spare bits, particularly at lower data rates. The adding of spare bits, however, takes up transmission path capacity. If the cellular radio system comprises a fixed rate vocoder, a frame transmitted in a DTX situation includes spare bits which do not contain any important information from the point of view of the user or the cellular radio system. The frame additionally contains comfort noise updating information transmitted at e.g. every 0.5 seconds. In addition, the prior art system employs a relatively long synchronization word which takes up capacity.